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Performance and Reliability

of Bonded Interfaces for

High-Temperature Packaging

Douglas DeVoto

Principal Investigator

National Renewable Energy Laboratory

Annual Merit Review and Peer Evaluation Meeting

June 17, 2014

Washington, D.C.

PR-5400-61357 Project ID: APE063
2

Overview

Timeline

• Project Start Date: FY14

• Project End Date: FY16

• Percent Complete: 10%

Barriers and Targets

• Cost

• Weight

• Performance and Lifetime

Budget

• Total Project Funding:

o DOE Share: $500K

• Funding for FY14: $500K

Partners

• Interactions / Collaborations

o Heraeus, Henkel, General Motors, 

Oak Ridge National Laboratory 

(ORNL) (Andrew Wereszczak)

• Project Lead

o National Renewable Energy 

Laboratory (NREL)
3

IGBT / Diode Die

Metalized Substrate

Substrate

Attach

Base Plate

Die Attach

Interconnect Encapsulant

Enclosure

Terminal

Relevance

• Current automotive power electronics are transitioning from silicon to 

wide bandgap (WBG) devices to meet cost, volume, and weight targets.

• Packaging designs must improve to take advantage of WBG devices’ higher 

junction temperatures (>200°C).

• Current bonded interface materials (BIMs) do not meet packaging 

requirements:

o Continuously operate above 200°C

o Meet RoHS standards (lead-free)

o Exhibit thermal fatigue resistance

o Provide high thermal conductivity.

Traditional Power Electronics Package

0 100 200 300

Btech HM-2

LF318

SAC305

SnPb37

Temperature (°C)

Operating Range

Above Operating Range

Reflow Range
4

Relevance

• As operating temperatures increase, the coefficient of thermal expansion 

(CTE) mismatch between the substrate and the base plate causes 

delamination initiation and propagation in the joining layer.

CTE (x 10-6 / K)

Silicon: 2.6

AlN: 4.5

Si3N4: 3.2

Copper: 16.5

Aluminum: 22.7

Sn63/Pb37 Solder: 24.7

Silver: 19.5

• Sintered-silver reliability has not been documented at 200°C conditions 

for the substrate attach layer.

o ORNL and NREL’s prior experience with sintered-silver processing will 

generate recommended practices for synthesis of reliable interfaces.

Cooling

Heating

Strain-Free Temperature
5

Milestones

2013 2014

Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep

Go/ 

No-Go

Key 

Deliverable

Model stress intensity factor with 

finite element analysis and fit V-K* 

curve to FY13 sintered-silver results.

Go/No-Go: Can threshold before delamination initiates be defined?

Key Deliverable: Publish delamination initiation findings for sintered-silver.

Process CTE mismatched 

disk samples with various 

diameter bond pads. ORNL

NREL Perform accelerated life testing 

and reliability characterization.

Process shear test 

samples.

*V= da/dN, Crack Growth Rate, (mm/cycle) – K = Stress Intensity Factor
6

Strategy

• Subject samples to shear tests for development of stress-strain curves.

• Focus on optimizing and understanding key synthesis parameters for 

sintered-silver.

1. Identify threshold at which stress intensity factors are sufficient to cause 

delamination initiation.

2. Evaluate the crack region where a transient delamination rate occurs.

3. Evaluate the crack region where a constant-slope delamination rate occurs.

• Develop stronger experimental correlation between interface patterning/ 

degradation and junction temperature rise.

0%

5%

10%

15%

20%

25%

30%

35%

0 1,000 2,000 3,000

Delamination

Number of Cycles

2

2.5

3

3.5

4

4.5

100 120 140 160

Distance across the 

Package (mm)

Temperature (°C)

New Degraded

1 2

3
7

Strategy

• Synthesize and shear test initial samples for mechanical characterization 

of sintered-silver.

o Attempt to measure residual stress at room temperature.

o Estimate stress-strain curves.

o Use information to model plastic deformation.

• Sintered-silver is bonded between two direct bond copper substrates.

• Process CTE-mismatched disk samples with various diameter bond pads 

to validate stress intensity factor relationship with delamination initiation.

o The stress intensity factor is a function of the loading amount, deformation mode, and 

the region of interest relative to the crack tip deformation.

o Crack tip deformation can propagate through three modes:

– Tension

– Shear

– Tearing.

CTE 1

Side Views

CTE 2

Top View

CTE 1

CTE 2

CTE 1

CTE 2
8

Strategy

• Identify threshold at which stress intensity factor is 

sufficient to cause delamination initiation.

o Model stress intensity factor with finite element analysis 

and correlate with FY13 sintered-silver results.

o Fit V-K curve to FY13 sintered-silver results. V= da/dN, Crack Growth Rate (mm/cycle)

KII, Shear Stress Intensity Factor

V-K Curve

K0 KC
9

CTE 2

CTE 1

Strategy

• Subject samples to accelerated temperature testing:

o -40°C to 200°C thermal shock

o 175°C and 250°C temperature elevation.

• Monitor delamination rates through acoustic microscopy.

-100

0

100

200

300

0 10 20 30

Temperature (°C)

Time (min)

Temperature Test Conditions
10

Strategy

• Evaluate the delamination rate of sintered-silver test coupons under 

various pressure requirements, bond areas, pad geometries, and surfaceplating materials.

Plating 

Material Ag, Au

Cleaning

None, substrate 

cleaning, preoxidation

Poor Ag Plating

0

5

10

15

20

25

Pressure (MPa)

Recommended Synthesis 

Pressure

Evaluate low- and no-pressure 

sintered-silver materials.

Optimize pad geometries for a 

large-area bond pad.

Recommend industry standard 

practices for plating.
11

Prior Temperature Cycling

Time

Failure Parameter

Time

Failure Limit

s1

s2

s3

s1<s2<s3

s3

s2

s1

t(s3) t(s2) t(s1)

-100

0

100

200

0 20 40 60 80 100

Temperature (°C)

Time (min)

Temperature Test Conditions

Low Ramp Rate

High Ramp Rate

• Thermoplastic film embedded with carbon 

fibers, sintered-silver, and Sn63/Pb37 solder 

were subjected to accelerated thermal cycling.

2,500 Cycles 2,500 Cycles 1,500 Cycles

Thermoplastic Sintered-Silver Solder

Technical Accomplishments

(Prior Work)
12

• After 2,500 thermal cycles, perimeter fracturing reached 19% to 32%.

2,000 Cycles

0%

5%

10%

15%

20%

25%

30%

35%

0 1,000 2,000 3,000

Delamination

Number of Cycles

Sample 1

Sample 2

Sample 3

Sample 4

Sintered-Silver Evaluation

Metalized Substrate

Base Plate

Sintered-Silver 91.6 μm

A

B

A B

Technical Accomplishments

(Prior Work)
13

BIM Degradation Summary

DeVoto, D., Paret, P., Narumanchi, S., Mihalic, M., 2013, “Reliability of Bonded Interfaces for Automotive Power Electronics,” InterPACK2013-73143, 

Proceedings of the 2013 InterPACK Conference, July 2013, Burlingame, CA.

Technical Accomplishments

(Prior Work)

0%

5%

10%

15%

20%

25%

30%

35%

0 500 1,000 1,500 2,000 2,500

Delamination

Number of Cycles

Sintered-Silver

PbSn Solder

Thermoplastic Adhesive
14

Interface Modeling

• A quarter symmetry model of the 

substrate test sample was created.

• Material parameters for a linear 

elastic analysis were applied to 

various layers of the model.

• Temperature is first raised to 

275°C, then lowered to room 

temperature. Quarter Symmetry Model

Model – Top View Sintered-Silver Layer – 5 mm Radius

Technical Accomplishments 

Lower Substrate

Upper Substrate
15

Interface Modeling

• Six linear elastic simulations were performed and incorporated variations in 

thickness and diameter of the sintered-silver joint.

• Stress increased with decrease in diameter of the sintered-silver joint for the 

same substrate size.

• Stress increased as thickness was increased from 25 μm to 50 μm.

Technical Accomplishments 

Thickness

(μm)

Diameter

(mm)

von-Mises 

Stress (MPa)

25 10 24.5

25 9 24.9

25 8 25.5

25 7 26.1

25 6 26.8

50 10 27.5

von-Mises Stresses in

Sintered-Silver Layer
16

Coupon Synthesis

• Invar and copper were selected for round test coupons.

o Coupon dimensions are 25.4 mm in diameter, 2 mm in thickness.

o Materials were chosen for CTE mismatch.

o Surfaces were blanchard ground and metalized with silver.

Technical Accomplishments

Invar and Copper Test Coupons

Invar Copper

Metalized 

Invar

Metalized 

Copper
17

Responses to Previous Year Reviewers’ Comments

• This is a new start for FY14.
18

Collaboration and Coordination

• Partners

o ORNL (Federal): technical partner on sintered-silver samples

o Heraeus (Industry): sintered-silver material guidance

o Henkel (Industry): sintered-silver material guidance

o General Motors (Industry): technical guidance
19

Proposed Future Work (FY14)

• Synthesize and shear test initial samples for mechanical characterization 

of sintered-silver.

• Process CTE-mismatched disk samples with various diameter bond pads 

to validate stress intensity factor relationship with delamination 

initiation.

• Focus on optimizing and understanding key synthesis parameters for 

sintered-silver.

• Develop stronger experimental correlation between interface patterning/ 

degradation and junction temperature rise.
20

Proposed Future Work (FY15)

• Evaluate the delamination rate of sintered-silver test coupons under 

various pressure requirements, bond areas, pad geometries, and surface 

plating materials.

o Evaluate low- and no-pressure sintered-silvers.

o Optimize pad geometries for a large-area bond pad.

o Recommend industry standard practices for plating.
21

Summary

• DOE Mission Support:

o BIMs are a key enabling technology for compact, lightweight, low-cost, 

reliable packaging and for high-temperature coolant and air-cooling 

technical pathways.

• Approach:

o Synthesis of sintered-silver bonds, thermal temperature cycling, bond 

inspection (acoustic microscope), and stress intensity factor versus cycles-tofailure models.

• Accomplishments:

o Established a procedure for the material and degradation characterization of 

sintered-silver.
22

Summary

• Collaborations

o ORNL, Heraeus, Henkel, GM

• Future Work

o Synthesize and shear test initial samples for mechanical characterization of 

sintered-silver.

o Process CTE-mismatched disk samples with various diameter bond pads to 

validate stress intensity factor relationship with delamination initiation.

o Focus on optimizing and understanding key synthesis parameters for 

sintered-silver.

o Develop stronger experimental correlation between interface 

patterning/degradation and junction temperature rise.
For more information, contact:

Principal Investigator

Douglas DeVoto

Douglas.DeVoto@nrel.gov

Phone: (303) 275-4256

APEEM Task Leader

Sreekant Narumanchi

Sreekant.Narumanchi@nrel.gov 

Phone: (303) 275-4062

Acknowledgments:

Susan Rogers and Steven Boyd

U.S. Department of Energy

Team Members:

Paul Paret (NREL)

Andrew Wereszczak (ORNL)